In recent years, research and development of a light-emitting element (organic EL element) that uses an organic compound and utilizes electroluminescence (EL) have been actively promoted. In a basic structure of such a light-emitting element, an organic compound layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying voltage to the element, light can be emitted from the light-emitting substance.
The light-emitting element is a self-luminous element and thus has advantages that the visibility of a pixel is higher than that of a liquid crystal display and that a backlight is not needed, and is considered suitable as a flat panel display element. In addition, it is also a great advantage that a display including the light-emitting element can be fabricated as a thin and lightweight display and has very fast response speed.
The light-emitting element can provide planar light emission. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element has great potential as a light source applicable to a lighting device and the like.
In such an organic EL element, electrons from a cathode and holes from an anode are injected into an EL layer. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is excited and provides light emission.
The excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S*:T*=1:3.
In a compound that emits light from the singlet excited state (hereinafter, referred to as fluorescent substance), at room temperature, generally phosphorescence is not observed while only fluorescence is observed. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent substance is assumed to have a theoretical limit of 25% based on the ratio of S* to T* that is 1:3.
In contrast, in a compound that emits light from the triplet excited state (hereinafter, referred to as a phosphorescent compound), phosphorescence can be observed at normal temperature. Since intersystem crossing (transfer of excitation energy from the singlet excited state to the triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be increased to 100% in theory. That is, a light-emitting element using a phosphorescent substance can have higher emission efficiency than a light-emitting element using a fluorescent substance. For this reason, light-emitting elements using phosphorescent compounds are now under active development in order to obtain highly efficient light-emitting elements.
A white light-emitting element disclosed in Patent Document 1 includes a light-emitting region containing a plurality of kinds of light-emitting dopants that emit phosphorescence. An element disclosed in Patent Document 2 includes an intermediate layer (a charge-generation layer) between a fluorescent layer and a phosphorescent layer (i.e., the element is what is called a tandem element).